Managing the world’s worst virus disease

The continued spread of leafroll virus in vineyards across California underscores the value of using certified virus-free rootstock at planting — the only way to prevent the costly disease.

Considered the most severe grape disease in the world, leafroll accounts for about 60 percent of grapevine losses caused by viruses. There is no cure and vines remain infected for life. It can slash the productive life of a vineyard by 50 percent or more.

Once thought to be on the decline, leafroll disease has on the increase in California vineyards over the past 10 years.

“It’s a much more severe problem on the North Coast than in the past, but we don’t know why,” says Deborah Golino, University of California Cooperative Extension plant pathologist and director of the UC-Davis Foundation Plant Services. “Some Central Coast growers have reported rapid spread of the disease, and there’s no reason to think that it’s not spreading in vineyards statewide.”

She has been researching leafroll for 20 years, and says, “I’m always surprised to find growers who haven’t heard about it before.”

It can cut yields by 10 percent to 20 percent, and can also lower sugar levels, discoloring the fruit, and delaying ripening by three to four weeks. This delayed maturity exposes grapes to rot-causing autumn rains.

It also has a long-term impact. In one five-year study of a Napa Valley Cabernet Sauvignon vineyard, the number of infected vines in one block increased an average of more than 10 percent annually, expanding from about 23 percent of the block in 2002 to just over 61 percent in 2006. Because of the lower quality of grapes produced by the infected vines, the vineyard needed to be replanted after just 15 years, half its expected productive lifespan, Golino says.

Leafroll disease is caused by at least 10 members of the Closteroviridae virus family. Of these, the major grapevine leafroll-associated virus threat in California is GLRaV-3, which is spread by mealybugs. The vine mealybug is considered the primary vector, but one or more or more of the others — the obscure, long-tailed, citrus and grape mealybugs — may be just as effective in carrying the virus to new areas. The soft scales found in California, vine scale and European fruit lecanium scale, may also be transmitting it.

Symptoms of leafroll virus — which can mimic other problems such as a fungal root disease, a nutritional deficiency, physical damage or feeding by pests — are most apparent between harvest and leaf fall. Starting with the basal leaf on the cane, the margins of the leaf blades roll downward, and areas between the major veins turn yellow or red, depending on the color of the grape variety. In some varieties, the area next to the major veins stays green until late fall. Some diseased vines may show no symptoms at all.

The only way to positively identify leafroll disease is to have samples of the vine tested by a competent diagnostic lab, Golino says.

One reason for the disease’s increase California might be the continued use by many growers of common rootstock that has not been checked for the virus. This includes simple rooted cuttings, infested with the virus, to propagate vines as well as the use of certified virus-free rootstock field-budded with scion wood bearing the disease.

“Even though the certified rootstock is free of the virus, the infected scion bud can transmit the virus to the entire vine, from top to bottom,” says Golino.

Another explanation for more infected vineyards may be an unknown virus strain that is transmitted more efficiently by mealybugs and other vectors.

There is a third possibility. “Perhaps leafroll viruses have always spread among vines in our vineyards,” Golino says, “but the symptoms simply were not evident in most cases, because the rootstocks were more disease-tolerant and showed fewer symptoms.”

She offers these tips for managing leafroll virus.

• Start a vineyard only with rootstock and scions that have been certified to be free of disease. A list of suppliers is available at http://ngr.ucdavis.edu

• Scout for mealybugs and symptoms of the disease.

• Keep detailed records. Document such details as when each variety was planted; when mealybugs were first seen and the types of treatments used to control them; when leafroll disease was first spotted, and how quickly the disease spreads from one year to the next. This information will be useful in determining your management plan.

• Develop your management plan. If the number of infected vines and spread of the disease remains low and you have kept the vectors under control, it may pay to continuing growing the vines. Otherwise, depending on the value of grapes, you may be money ahead in the long run to rip out the vineyard and replant.